The present invention relates to an ultrafine powder of silicon carbide, a method for the preparation thereof and a sintered body therefrom. More particularly, the invention relates to an ultrafine powder of silicon carbide composed of spherical agglomerate particles having an average diameter of 0.01 to 1 .mu.m formed of crystallites of a size of 5 nm or smaller and a method for the preparation of such an ultrafine powder of silicon carbide by the method of vapor phase pyrolysis of a vaporized specific organosilane compound under specific reaction conditions as well as a sintered body obtained from such an ultrafine powder of silicon carbide.
As is well known, silicon carbide is a very promising material having extreme stability against chemical and physical influences and excellent properties such as, in addition to the resistance against oxidation and attack of chemicals, high heat conductivity, low thermal expansion coefficient, high mechanical strengths including hardness so that silicon carbide is used for manufacturing high performance articles such as blades of gas turbines, parts of automobiles, high corrosion-resistant or high heat-resistant refractory parts in chemical plants, electronics materials and the like.
These shaped articles of silicon carbide are usually manufactured by a method of sintering in a variety of processes. Some of the processes in the prior art include (1) a method of reactive sintering or infiltration in which a green body shaped of a powdery mixture of silicon carbide and carbon or a calcined body thereof is brought into contact with molten silicon to infiltrate therethrough so that the molten silicon reacts in situ with the carbon in the shaped body to be converted into silicon carbide, (2) a method of pyrolysis of an organosilicon compound at a high temperature on a suitable substrate of a previously sintered silicon carbide body to deposit silicon carbide as the pyrolysis product thereon and (3) a method by use of a sintering aid in which a fine powder of silicon carbide is admixed with a sintering aid such as metallic aluminum or oxide of aluminum, a combination of carbon and elementary boron and the like and the powdery mixture is shaped into a form and sintered by heating under normal pressure or under a superatmospheric pressure.
A problem in the first mentioned reactive sintering method is that the finished body more or less contains of necessity certain amounts of unreacted silicon so that the sintered body has poor resistance against alkali and relatively low mechanical strengths, especially, at high temperatures. The silicon carbide shaped body obtained by the above mentioned second method is disadvantageous when high mechanical strengths are desired since the vapor phase pyrolysis of the orgnosilicon compound usually cannot reach the depth of the substrate body. Further, the usability of the sintered body of silicon carbide obtained in the third method by use of a sintering aid is naturally limited by the temperature not exceeding the melting point of the sintering aid with great decrease of the mechanical strengths at higher temperatures. In particular, although a sintered body of a relatively high density can be prepared by use of a combination of carbon and elementary boron as a sintering aid, the effective amount of the boron is so large as to sometimes exceed 0.15% by weight so that the silicon carbide sintered body prepared in such a method is quite unsuitable when a high purity is essential as in the articles used in the processing of semiconductor materials due to the presence of free boron on the grain boundaries. This method is of course a good method if the amount of the sintering aid can be greatly decreased if not totally omitted but no powder products of silicon carbide are known hitherto in the art which could be sintered without or with a greatly reduced amount of a sintering aid.
Turning now to the method for the preparation of a silicon carbide powder suitable for sintering, several methods are known in the art. The most basic or traditional method is (1) the pulverization of silicon carbide obtained by the high temperature reaction of elementary silicon and carbon in an electric furnace. Alternatively, methods are known in which (2) a silicon carbide powder is formed by the vapor phase pyrolysis of an organosilane of the formula R.sub.n SiX.sub.4-n, in which R is a hydrogen atom or an alkyl group, X is a halogen atom and n is an integer of 1 to 4, or a gaseous mixture of the silane with a hydrocarbon compound, (3) a high polymeric polycarbosilane compound is pyrolyzed to form silicon carbide or (4) silicon dioxide is reacted with carbon at a high temperature to form silicon carbide which is then pulverized.
The above mentioned first method is suitable when .alpha.-type silicon carbide is desired but practically disadvantageous because the method involves the process of pulverization of very hard silicon carbide so that no fine powdery product of high purity can be obtained in a high yield. The second method is disadvantageous, although a relatively fine silicon carbide powder of .beta.-type can be obtained by the method, due to the very high reaction temperature required for the pyrolysis. In addition, the powdery silicon carbide product obtained by this method necessarily contains a small amount of silicon-bonded halogen atoms which adversely affect of the sintering behavior of the powder if not to mention the problem of the disposal of the hydrogen halide such as hydrogen chloride formed as a byproduct of the reaction. The problems in the third method are the expensiveness of the polycarbosilane compounds and the necessity of the pulverization of the pyrolysis product into a fine powder. The fourth method is also disadvantageous due to the high temperatures required for the reaction and the necessity of the pulverization process. In addition, the silicon carbide product formed by this method always contains unreacted starting materials and elementary silicon as a byproduct so that a purification treatment of the silicon carbide product is indispensable. Furthermore, the particle configuration of the silicon carbide powder obtained by a method involving a pulverization process is naturally not spherical and the particle size distribution thereof is also not uniform so that the sintering behavior of such a pulverized silicon carbide powder is usually not satisfactory.